1. Field of the Invention
The present invention relates to a tracking monitoring device for determining the state of how a weld line is tracked under arc sensing control to let the oscillating center of a welding torch track a correct weld line along the groove center based on a welding current waveform when the welding torch is oscillated in the direction of the groove width, and a tracking control device using the monitoring technique.
2. Detailed Description of the Related Art
In recent arc welding systems, as the output control of the welding power supply has been shifted from the thyristor type to the inverter type, so that the control speed has been increased from 300 Hz to 15-60 kHz, thereby to be 50-200 times faster than the prior apparatuses. Moreover, the waveforms control of the welding current and welding voltage can be made. Thus, the arc starting property and the welding stability during a high speed arc welding may be improved, while the amount of spatters produced can be reduced, so that the welding stability during an arc welding has been also improved.
Meanwhile, as a method of controlling a welding torch to track a weld line, an arc sensing control is generally used. In the arc sensing control, based on change in the welding current associated with change in the wire extension length when the welding torch is oscillated in the direction of the groove width, the positional correction (moving the center of oscillation in the horizontal and vertical directions) is made in the groove width direction of the welding torch (hereinafter referred to as xe2x80x9cthe oscillating directionxe2x80x9d) and the groove vertical direction (hereinafter referred to as xe2x80x9cthe direction of the torch axisxe2x80x9d).
The arc phenomenon is very unstable and proceeds at an ultra high speed, and therefore the reliability of the present arc sensing control is limited. In addition, an objective methodology to determine the quality of welding has not been developed yet, and defectively welded products could be unexpectedly marketed at any time. Since the reliability of the present arc sensing control is low and in the fields requiring high quality of arc welding such as high speed welding for a thin steel plate, more complicated and expensive active control systems other than the arc sensing control such as providing a laser displacement sensor at the top of a welding torch for example is inevitable.
Therefore, various methods to improve the reliability of the arc sensing control which is simple and easy have been proposed, but anything satisfactory in practical level has not yet been suggested.
According to the method disclosed by Japanese Patent Publication No. Sho 53-11502 for example, the magnitudes of the integrated value of welding current at prescribed intervals obtained at both ends of weaving (oscillation) are compared each other mainly for adjusting displacement in the groove width direction. The welding torch has then its weaving center position corrected in the transverse direction based on the comparison signal and is made to track the correct weld line.
According to this method, however, the integrated value of welding current at both ends of weaving must be averaged several times in order to improve the positional detection accuracy, the output of the positional excursion correction signal is delayed by a time period corresponding to the number of averaging, and the tracking control accuracy of the welding torch is degraded. If the welding speed is slowed to shorten the weld line proceeding during this wasteful time, the tracking control accuracy certainly improves and yet the production efficiency is downed instead. In addition, in order to prevent the output of the positional excursion correction signal from being delayed without lowering the welding speed, the weaving frequency may be increased. If however the weaving frequency is increased, the weaving frequency is reduced, integrating and averaging the welding current including much disturbance information does not improve the removal ratio of the disturbance information. If the number of averaging is increased, the adverse effect of the disturbance information upon the correction signal may be reduced, but the output of the positional excursion correction signal is delayed for the increase in the number of averaging which would eventually prevent the tracking control accuracy from improving.
The method proposed by Japanese Patent Publication No. Hei 2-4396 is directed to correction of positional excursion from a weld line in weaving welding process. More specifically, there are provided a comparator to identify whether it is in a short-circuit period or in an arc period based on a welding voltage signal for the arc period, a selecting circuit to extract a welding current signal in the process of arc duration by a signal identifying arc duration from the comparator, an integrator to integrate the output signal of the selecting circuit for one cycle or half the cycle of weaving, a counter to count arc period, an average value calculation circuit to output an average value signal produced by averaging the outputs of the integrator by the arc period, and a hold circuit to hold the average value signal at the end of one cycle or half the cycle of weaving. Here the output of the hold circuit and a set value for welding current are compared to determine a positional excursion in the vertical direction, i.e., the direction of the welding torch axis. Furthermore, a positional excursion in the horizontal direction, i.e., the direction perpendicular to the direction of the welding torch axis is further determined based on the polarity of the output of the hold circuit. A positional excursion correction instruction is then output to a correction motor.
According to the method, however, if a set value for welding current is changed during an arc welding, a distance between contact tip and base metal set prior to the start of welding is changed, and arc welding cannot be surely performed.
According to the method proposed by Japanese Patent Publication No. Sho 57-2428, the welding current at both ends of weaving is detected and compared to a prescribed reference value to obtain a signal for positional excursion correction from the weld line.
According to the method, however, the detected welding current contains much disturbance information other than the positional information, and the position detection accuracy can be extremely low in a low current state such as in a short-circuiting transfer state and a globular transfer state. The method is therefore only limitatively applicable in a high current, spray transfer state.
According to the method proposed by Japanese Patent Publication No. Hei 7-4666, assuming that the arc length is fixed regardless of the position of oscillation, the ratio of the integrated values of welding current in the first half and the second half in the regions of the right and left halves of the oscillating width in one cycle of oscillation (first and second comparison signals) are operated. The oscillating center of the welding torch is then moved in the direction in which the difference between these comparison signals is reduced.
By this method, however, extremely unstable waveforms of welding current in a short-circuit period are incorporated as they are into the integrated values of welding current. Therefore, it has been confirmed that even if the welding torch actually normally tracks with no positional excursion in operation, and sputtering is stable, the first and second comparison signals randomly make great leaps. Therefore, if the welding torch is controlled to be moved in the direction in which the difference between the first and second comparison signals is reduced, the welding torch could depart from the weld line on the contrary. Furthermore, the method is only on condition that the arc length is approximately fixed, while in practice the welding wire melting speed sometimes cannot follow changes in the welding current with time by the influence of the inductance of the welding power supply circuit. Thus the welding torch can not be controlled to accurately be moved due to the change of the arc length.
It is an object of the present invention to provide a tracking monitoring device and a tracking control device for a weld line less affected by a short-circuit mode or sputtering mode which serves as a disturbance factor in a welding current value and capable of coping with a welding speed increase in association with higher speed oscillating frequency.
In order to achieve the above object, the tracking monitoring device for a weld line according to the present invention includes:
(A) welding current detecting means for detecting a welding current waveform obtained when a welding torch is oscillated in a groove width direction, wherein a short-circuit period and arc period are repeated for groove welding in the consumable electrode gas shield arc welding;
(B) short-circuiting current waveform removing means for removing a short-circuiting current waveform from the welding current wave form detected by the welding current detecting means, wherein the short-circuiting current waveform is a welding current waveform in a short-circuit period;
(C) interpolating means for linearly interpolating a missing part in the welding current waveform removed of the short-circuiting current waveform;
(D) sampling means for sampling the linearly interpolated welding current waveform at a plurality of intervals corresponding to the oscillating frequency of the welding torch;
(E) replacing means for replacing the sampled welding current waveforms by an approximated straight line by a least square method or a substantially equivalent method thereto; and
(F) oscillating direction positional excursion display means for displaying the positional excursion direction and positional excursion distance of the welding torch in the oscillating direction relative to a groove central position as a reference, wherein the positional excursion direction and positional excursion distance correspond to the inclination direction and inclination angle of the approximated straight line.
The sampling means includes folding and superposing means for sampling a linearly interpolated welding current waveform at a plurality of intervals corresponding to one cycle of oscillation of the welding torch, and folding and superposing the sampled welding current waveforms at a position corresponding to half the cycle of oscillation. When the superposed welding current waveforms are replaced by an approximated straight line by the replacing means, a height at a fixed point corresponding to the central position of the approximated straight line, in other words, a point corresponding to a position of one fourth (90xc2x0) to three fourths (270xc2x0) of the cycle of the oscillation of the welding torch and a predetermined set value for welding current may be compared to display the difference as the positional excursion direction and positional excursion distance of the welding torch in the direction of the welding torch axis. Note that though rare if a groove center is not particularly taught in the case of different plate thickness welding for example, the position at the fixed point might have to be set other than at a position corresponding to one fourth to three fourths of the cycle of oscillation.
When the welding current waveform is replaced by an approximated straight line by the replacing means, the welding current waveform may be previously converted into a smoothed current waveform using average means according to a moving-average method. Thus, indented waves in the welding current waveform created by sputtering or the like can be eliminated, and disturbance information other than the positional information of the welding torch can be removed as much as possible. At the same time, by setting the waveform sampling cycle to be relatively long at the time of replacement to the approximated straight line, sufficient tracking control accuracy in practice can be maintained as the operation speed is to be further increased.
More specifically, the short-circuiting current waveform removing means includes threshold value setting means for setting a reference welding voltage to differentiate an arc period and a short-circuit period in a welding voltage waveform as a threshold value. However, though for short-circuit period, there still remains a large disturbance factor other than the positional information of the welding torch at a part immediately before the start of a short-circuit period and a part immediately after the start of an arc period, even in a welding voltage region higher than the threshold value or reference welding voltage. Thus, it is preferred that a waveform portion to be cancelled as a short-circuit period is extended backward and forward by the first operation means and the second operation means, and the welding current waveform including the part immediately before the start of a short-circuit period and the part immediately after the start of an arc period is canceled. As a result, the influence of the short-circuit period as a large disturbance factor can be substantially completely removed in monitoring the tracking of the welding torch.
Further, the tracking control device for a weld line includes:
(A) welding current detecting means for detecting a welding current waveform obtained when a welding torch is oscillated in a groove width direction, wherein a short-circuit period and arc period are repeated for groove welding in the consumable electrode gas shield arc welding;
(B)short-circuiting current wave form removing means for removing a short-circuiting current waveform from the welding current waveform detected by the current detecting means, wherein the short-circuiting current waveform is a welding current waveform in a short-circuit period;
(C) interpolation means for linearly interpolating a missing part of the welding current waveform removed of the short-circuiting current waveform;
(D) sampling means for sampling the linearly interpolated welding current waveform at a plurality of intervals corresponding to the oscillating frequency of the welding torch;
(E) replacing means for replacing the sampled welding current waveforms by an approximated straight line by a least square method or a substantially equivalent method thereto;
(F) oscillating direction excursion operation means for performing operation to obtain the positional excursion direction and positional excursion distance of the welding torch in the oscillating direction relative to a groove central position as a reference, wherein the positional excursion direction and positional excursion distance correspond to the inclination direction and inclination angle of the approximated straight line; and
(G) the direction of the welding torch axis positional excursion operation means for comparing a height at a fixed point on the approximated straight line corresponding to one fourth and three fourths of the oscillating frequency of the welding torch to a set value for welding current, thereby performing operation to obtain the positional excursion direction and positional excursion distance of the welding torch in the direction of the welding torch axis, and
(H) based on the operation results of the oscillating direction positional excursion operation means and the direction of the welding torch axis positional excursion operation means, the oscillating center of the welding torch and the height of the torch are controlled to move in a direction to cancel the oscillating positional excursion direction and the positional excursion distance of the direction of the welding torch axis. Similarly to the tracking monitoring device described above, the averaging means and folding and superposing means can be applied to this tracking control device.